1. Field of the Invention
The invention relates to the use of a chromium-containing, martensitic iron-based alloy for plastic molds.
2. Discussion of the Background of the Invention and Material Information
Iron-based alloys with a chromium content of more than 12% are generally used in the production of corrosion-resistant plastic molds for processing chemically-reactive molding compounds. Depending on the required (or desired) hardness of the material, heat-treatable Cr steel with 13.0% Cr and 0.2 or 0.4 weight-% C, for example in accordance with DIN Material Number 1.2082 and 1.2083, are employed. These iron-based alloys essentially containing carbon and chromium are easily and economically usable for less stressed molds, but they have the disadvantage that a sufficient service life of the mold (or tool) cannot be attained when subjected to highly corrosive molding compounds with wear-causing additives.
Iron-based alloys, for processing of plastics, which are more corrosion-resistant can be obtained by increasing the chromium content to 14.5 weight-% and increasing the carbon content to 0.48 weight-%, and adding 0.25 weight-% of molybdenum in accordance with DIN Material Number 1.2314. In practical use, such materials are mostly sufficiently resistant to chemical reactions but have, particularly in connection with molding materials containing mineral fibers, insufficient resistance to wear.
Improved use properties of plastic molds with respect to oxidation/corrosion and wear can be attained by comparably large chromium contents, large carbon contents and molybdenum and vanadium content of the steel used. The material No. 1.2361 in accordance with DIN constitutes an iron-based alloy typical for this use in connection with highly-stressed plastic molds. However, in the course of producing molds or tools from this alloy, it is possible that material distortion or uneven dimensional changes may occur, which often requires expensive finishing work or discarding of the worked part. As one skilled in the art knows, such uneven dimensional changes are essentially caused by a deformation texture or a linear arrangement of the carbides. If now, as has been proposed in the prior art, the carbon content and thus the carbide portion in the matrix is reduced, the wear resistance of the material in particular is also reduced. Further, the wear of the mold under large directional stress is increased and the service life reduced. A further disadvantage of a high carbon content is the low stretching ability and the reduced toughness of the steel.